The present invention relates to an electrophotographic image forming apparatus applicable to apparatuses which form monochrome images and color images, such as copying machines, facsimile machines, printers, and compound machines with these functions combined, and a method for cleaning an image carrier belt in the image forming apparatus.
In electrophotographic image forming apparatuses, as publicly known, image formation is performed by forming an electrostatic latent image by exposing the surface of a charged image carrier (e.g., drum-shaped or endless belt-like photoconductor), developing the electrostatic latent image with toner particles supplied from a developing device to form a toner image on the photoconductor, and transferring the toner image onto a paper sheet with a transfer device. As the transfer device, those having an endless belt-like image carrier belt (so-called intermediate transfer belt) are well known. In such type of transfer device, a compound color image is formed on the intermediate transfer belt by serially transferring (primarily transferring) toner images of respective colors on the photoconductor onto the intermediate transfer belt by a primary transfer roller, and transferring (secondarily transferring) the color image onto a paper sheet by a secondary transfer roller.
Since untransferred toner particles may remain on the surface of the photoconductor or the intermediate transfer belt after the transfer process, it is necessary to remove the residual toner prior to the next image formation process. As a means (cleaning means) to remove and clean up such transfer residual toner and the like from the surface of the photoconductor or the intermediate transfer belt, a method for scraping transfer residual toner and the like from a target surface using a cleaning blade made of elastic materials such as rubber or soft resin is generally and widely adopted as it is inexpensive and is easy to operate.
In the case of scraping and cleaning the transfer residual toner and the like on the image carrier belt such as endless belt-like photoconductors (photoconductor belts) and intermediate transfer belts with the cleaning blade, a sliding contact portion (blade edge) of the cleaning blade with the belt surface may wear as it rub against the advancing belt surface, and this wear piece may grow in the state of being adhered to the blade edge. This tendency is particularly notable when the hardness of the blade material is relatively low and so-called SP (solubility parameter) values of the blade material and the belt material are close. Sometimes, the wear piece may grow up into a size of about ten micrometers.
The growth of such a blade wear piece occurs downstream of the blade edge in the belt movement direction, i.e., on a blade edge rear-face side opposite to the blade edge front face side where scraped transfer residual toner is accumulated.
If the grown-up wear piece is caught between the blade edge and the belt surface, the sliding contact pressure of the blade edge applied to the belt surface becomes uneven in a belt width direction, thereby causing a problem of cleaning failure such as toner particles passing through.
For example, JP 10-10939 A, JP 2005-3983 A and JP 2001-350384 A, which do not directly discuss the problem of adhering and growth of the blade wear piece, disclose driving the belt in the direction opposite to the normal direction under fixed conditions in order to remove matter such as toner, paper powder and talc attached or deposited on the top end of the cleaning blade or its back side.
In the image forming apparatus disclosed in JP 10-10939 A, it is stated that a photoconductor belt or an intermediate transfer belt as an image carrier is moved backward after termination of rotation at the end of image formation, so as to prevent the rotation of the belt from stopping in the state that toner, paper powder and the like are adhered and accumulated on the top end of the cleaning blade, as a result of which the cleaning performance of the cleaning blade can be maintained in approximately the initial state.
However, although the image forming apparatus disclosed in JP 10-10939 A can prevent the rotation of the belt from stopping in the state that toner, paper powder and the like are adhered and accumulated on the tip (blade edge) of the cleaning blade, the adhered and accumulated toner, paper powder and the like are only released from the cleaning blade by the belt reversing drive, and there is no means to actively remove these toners, paper powder and the like from the belt surface at the time of belt reversing operation. Therefore, in this case, the toners, paper powder and the like released from the cleaning blade by the belt reversing drive are to be removed when the belt is transported again to the position facing the cleaning blade. In this structure, it is impossible to remove the blade wear piece grown in the state of being adhered to the blade edge on the downstream side of the blade edge in the regular belt movement direction.
In the image forming apparatus disclosed in JP 2005-3983 A, the photoconductor belt or the intermediate transfer belt as an image carrier is driven in the regular direction and then is temporarily driven in the reverse direction before being driven again in the regular direction. During the time the belt is driven in the reverse direction and then is again driven in the regular direction to a start position of the reverse driving, the belt and the cleaning blade are relatively moved in the belt width direction. It is stated that even if the cleaning blade has slight defects such as chips, adopting this structure can prevent the toner and paper powder on the belt from remaining in the part of the slight defects without being scraped off.
However, in this case, the belt is simply driven in the reverse direction and the belt and the cleaning blade are relatively moved in the belt width direction. Consequently, it is impossible to remove the blade wear piece grown in the state of being adhered to the blade edge on the downstream side of the blade edge in the regular belt movement direction.
Further, the image forming apparatus disclosed in JP 2001-350384 A describes the structure in which an intermediate transfer belt as an image carrier is put in pressure contact with a cleaning blade in the vicinity of the upstream of a belt driving roller. It is stated that by adopting this structure, a contact part of the belt with the blade edge is deformed in the state of being sagged due to momentary drop of the belt tension at the time of belt reverse rotation, and as the belt is reversed in this state, the rear face side of the blade edge is rubbed against the belt surface, so that talc and the like adhered and deposited on the rear face side of the blade edge can be removed.
However, it is only an instant at the time of belt reverse rotation that the rear face side of the blade edge is rubbed against the belt surface. Moreover, the rubbing force for rubbing the blade edge by the belt surface is not very strong as it simply uses the sagging of the belt. Therefore, it is insufficient for removing the blade wear piece grown in the state of being adhered to the blade edge on the downstream side of the blade edge in the regular belt movement direction.
As mentioned above, in any of the conventional technologies, it was impossible to ensure removal of the blade wear piece grown in the state of being adhered to the blade edge on the downstream side of the blade edge in the regular belt movement direction, which posed a problem of difficulty in preventing cleaning failure due to the wear piece caught between the blade edge and the belt surface.